This project will investigate several aspects of the chemistry and physiology of pepsinogens. Chemical properties to be studied will be the effect of chemical modification of the activation peptide on its ability to act as an inhibitor of pepsin (and possibly other gastrointestinal proteases), and to resist pepsin degradation. These modified peptides will be tested against porcine, bovine and canine pepsins. Canine pepsinogens will be fractionated and the amino acid sequences of the three individual pepsinogens will be determined for structural comparisons among themselves and with the porcine and bovine zymogens. Concurrent with this, methods will be developed for convenient, practical C-terminal sequence analysis of peptides. Antibodies will be raised to the three distinct canine pepsinogens to permit detection of these proteins in different parts of the stomach, and their appearance on stimulation of gastric secretion in the blood and gastric juice of test animals. Other aspects of this project are (a) the use of small peptide substrates to define differences in specificities of pepsinogens from different species and the different pepsinogens from the same species, and (b) the isolation and characterization of rainbow trout pepsinogen.